"Invasion of privacy" is a complaint which may no longer be limited to outraged movie stars. With so many electronic instruments on the market these days which can surreptitiously pick up a personal conversation, the eavesdropper has a big advantage.

Tiny battery-powered microphone can easily be concealed under gown and is a natural for television and sound-stage work. Some of the listening devices cost $1,000.

Some of the devices are so small that they can easily be hidden in a breast pocket. They are powerful enough, however, to beam a high-fidelity signal more than a quarter of a mile.

The instruments are being put into legal use by television, movie, industrial and official circles. They are a natural for TV programs like "Person to Person" and for police work.

Compact mike and power unit installed under dashboard will not only relay conversation but also the car's speed and direction to a following auto.

Monitor stationed blocks away from scene can preserve conversation by connecting receiver to tape recorder. This type of eavesdropping is especially useful to law enforcement agencies.

Power unit is not much bigger than the transmitter. Chances of detection are small when it is strapped to leg and hooked to mike at waist.